Microporous membranes can be used as battery separators in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous membranes are used as battery separators, particularly as lithium ion battery separators, the membranes' performance significantly affects the properties, productivity and safety of the batteries. Accordingly, the microporous membrane should have suitable mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc. It is desirable for the batteries to have a relatively low shutdown temperature and relatively high temperature stability for improved battery-safety properties, particularly for batteries that are exposed to high temperatures during manufacturing, charging, re-charging, overcharging, use, and/or storage. Improving separator permeability generally leads to an improvement in the battery's power and capacity. Low shutdown temperature is desired for improved battery safety, particularly when the battery is operated under overcharge conditions. Improved separator storage stability is desired because deterioration of separator at high temperature causes decreased battery voltage.
Microporous membranes comprising polyethylene having an amount of terminal unsaturation of more than about 0.2 per 10,000 carbon atoms have been disclosed for use as battery separators. These separators have a relatively low strength, but also provide a low shutdown temperature, which leads to improved battery safety as disclosed in WO1997/23554A and JP2002-338730A. Microporous membranes comprising polyethylene having such an amount of terminal unsaturation, however, have also been observed to deteriorate during battery storage and use. It is believed that the deterioration results at least in part from polyethylene oxidation reactions. Microporous membranes comprising polyethylene having an amount of terminal unsaturation of less than about 0.2 per 10,000 carbon atoms have also been disclosed as useful for battery separators. Batteries containing these separators show less deterioration during battery storage and use, but these batteries have a higher shutdown temperature.
Consequently, it would be desirable to provide a multilayer polyethylene membrane with an improved balance of shutdown temperature and oxidative resistance.